(1) Field of the Invention
The present invention relates to a developing device and an image forming apparatus using the device, in particular relating to a developing device using a dual-component developer containing a toner and a magnetic carrier, for use in an image forming apparatus for forming images with the toner based on electrophotography, such as an electrostatic copier, laser printer, facsimile machine or the like, as well as to an image forming apparatus using this device.
(2) Description of the Prior Art
Conventionally, image forming apparatuses based on electrophotography such as copiers, printers, facsimile machines and the like have been known. The image forming apparatus based on electrophotography is constructed so as to form an image by forming an electrostatic latent image on the photoreceptor drum (toner image bearer) surface, supplying toner to the photoreceptor drum from a developing device to develop the electrostatic latent image, transferring the toner image formed on photoreceptor drum by development to a sheet of paper or the like, and fixing the toner image onto the sheet by means of a fixing device.
Recently, in the image forming apparatuses capable of reproducing full-color and high-quality images, a dual-component developer (which will be referred to hereinbelow as simply “developer”), which enables the toner to present excellent charge performance stability, is often used.
This developer consists of a toner and a carrier, which are agitated in the developing device and frictionally rubbed with each other so as to produce appropriately electrified toner.
In the developing device, the electrified toner is supplied to a developer supporting member, e.g., the surface of a developing roller. The toner thus supplied to the developing roller is moved by electrostatic attraction to the electrostatic latent image formed on the photoreceptor drum. Hereby, a toner image based on the electrostatic latent image is formed on the photoreceptor drum.
Further, the image forming apparatus of this kind is demanded to be made compact and operate at high speeds, it is hence necessary to electrify the developer quickly and sufficiently and also convey the developer quickly and smoothly.
To deal with such demands, a developing device of a circulating mechanism has been adopted in the image forming apparatus in order to disperse added toner promptly into the developer and provide the toner with a suitable amount of static charge. This circulating type developing device includes a developer conveying passage through which the developer is circulatively conveyed and a developer conveying member that conveys the developer while agitating the developer in the developer passage (see Patent Document 1: Japanese Patent Application Laid-open H10-63081)
In this circulating type developing device, when the toner concentration in the developer in the developing device becomes lower than a predetermined level, toner is added from the toner hopper to the developer conveying passage.
However, since the conventional circulative type developing device conveys the developer whilst mixing (agitating) the added toner with the existing developer, if the agitating force is low there occurs the problem that the toner is conveyed to the developer roller without gaining a sufficient amount of static charge.
On the other hand, if the performance of agitating the developer and toner is enhanced by providing mesh-like screen members or arranging many parts such agitating paddles etc., there occurs the problem that the developer receives much stress and hence is lowered in durability.
Particularly, for a developer containing micro-sized carrier and a micro-sized toner, if the agitating force is enhanced, toner fluidity enhancer particles (external additive) become embedded into the toner surface due to stress, and the fluidity of the developer lowers extremely, making it difficult to convey the developer. As a result, the necessary amount of toner cannot be supplied to the photoreceptor drum surface, causing a problem of the image density being lowered.
Further, since, in the developing device in which the developer is conveyed as being agitated, the specific gravity of the developer (carrier) is very high (about three times) relative to the specific gravity of the added toner, it is difficult for the added toner to fall down into the bottom of the developer. As a result, there occurs the problem that the toner is conveyed to the developer roller without gaining a sufficient amount of static charge. If the agitation performance is enhanced by use of agitating paddles, the mixing performance (static charge performance) may be improved but there occurs the problem that the fluidity of the developer becomes markedly lowered due to stress from agitation.